The mask is both a cover for the face and a language that the face speaks. It disguises; it demarcates boundaries; it opens gates of passage; it threatens and celebrates. The study of the multiple functions of masking occupies many disciplines, including Anthropology, art and literary criticism, psychology and sociology; and complex patterns of arguments about the mask's various functions have been accumulated over the centuries. The mask, in its multifaceted glory and significance, is a central thread in the story of the human species.
The first law in any attempt to understand its use in fantasy is that, unless force has been applied, a mask does not remain fixed in place. It may be the covering that conceals a face of flesh (or, as often in fantasy, not-flesh) or it may be the face that the covering hides; but when it represents a covering the movement of the text will be to uncover what is hidden, and when the mask is an empty shell the movement of the text will be to ensure that it soon covers a countenance. At any one point in a tale, a mask means more than it apparently does; for, in literature, a mask almost always contains (and threatens to reveal) its opposite. To the reader, the introduction of a mask into a text inevitably arouses an expectation of change.
Very roughly, masks can be thought of as agents of transition, transformation, bondage or aspect-maintenance.
Transition The transitions signalled by masks can be further broken down roughly into three categories.
(a) When a transition is registered from the viewpoint or agency of any character with whom the reader is identifying, that transition can be seen as a passage from here to beyond. Masks in this sense represent (and in fantasy terms clearly are) Portals that enable significant Thresholds to be crossed: between human and animal; between human and Gods and Demons; between this world and Faerie or any other Alternate World (a task signalled, along with other forms of metamorphic transition, by the ten masks featured in the first half of Robert Holdstock's Lavondyss , part of the Ryhope Wood cycle); between the present and the past (> Time Abyss); and between the present – often via a Night Journey – and the future Eucatastrophe which resolves the tale. The crossing of these various thresholds is fundamental to the Quest structure of most Genre Fantasy, and when masks are invoked in this kind of tale they are, therefore, most often seen as enabling devices.
(b) When the reverse is the case – when the passage is through the mask from beyond to here – masks represent (and, especially in Horror, clearly are) portals between the animal and the human (as in the masks worn by various Mythagos in Holdstock's Ryhope Wood books); between God or Avatar or Malign Sleeper and human (as in Arthur Quiller-Couch's "Oceanus" , in which all the suffering humans in the great arena washed by God's tears wear masks which display one face only, that face being Christ's); between other Realities and this world (as in A.E.'s "The Mask of Apollo"  and Roger Zelazny's and Thomas T Thomas's The Mask of Loki ); between the past and the present; between the primitive and the civilized (as in H Rider Haggard's The Yellow God  and innumerable other stories in which "native" masks encroach upon the boundaries that define civilized realities); and – unusually – between the future and the present.
(c) When the passage represents a movement of the world itself across a threshold, then the fantasy deployments of the mask merge with traditional patterns of usage throughout human culture. Any use within a fantasy text of masks to mark a passage of Seasons may therefore generate a sense that the "real" world is being quoted, and that the mask may be in this context of less intrinsic significance in understanding how the particular story intends to proceed. In the end, we should perhaps keep in mind that, in human cultures, masks have at least two functions: they act to mark and to enable the passage, to make it liminal, so that it can be noticed and celebrated; and by their fixity they emphasize a pre-Christian sense that seasons and history (and the patterns of individual lives) return upon themselves, that they comprise Cycles – that the Story returns.
Transformation When the threshold is within the self and the sense of literal passage is therefore rendered metaphorical, masks represent a Transformation, like that enjoyed by the protagonist of The Mask, either in comic-book form or as filmed (> The Mask ) or by Uther Pendragon in the movie Excalibur (1981), when Merlin transforms him into his enemy (so he can seduce Ygrayne, his enemy's wife), a magical operation signalled when the mailed helmet covering Uther's face changes suddenly into another; or the transition from illness to health (> Fisher King) or vice versa; or the imposition of that internal change from an outside source, as in any story featuring a Medusa figure (> Gorgons) or a Totem-derived imposition of "true" being on a character, as in Lisa Goldstein's A Mask for the General (1987), Gloria Hatrick's Masks (1992) or Stephen Marley's Mortal Mask (1991); or the bearing, by human or god, of both poles of transformation simultaneously (> Face of Glory), an image conveyed by any mask which can be understood right-side-up or upside-down, or any double mask which laughs or weeps depending on which way it is turned (>>> Janus; Trompe-L'oeil).
Bondage There is always a danger that the passage will not succeed, that the real – and perhaps abominable – visage will remain locked beneath the mask, until the pressure grows too great; it is a pressure of this sort that gives Edward Lear's "The Dong with the Luminous Nose" (1877) its intensity of pathos, as the nose is a mask which disguises the poet himself. The pressing of a loathsome reality against a mask is a central motif of Horror and defines much of the fantasy appeal of Gaston Leroux's Phantom of the Opera.
In fantasy any mask with a frozen grimace showing signs of tears is certain to have been presented consciously as a sign of bondage, as a sign of a being (or succession of beings, as in Henry Kuttner's The Mask of Circe [1948; 1971], where generations of Circes are frozen into one identity by a single mask) which has become fixed. Malign Sleepers may on being awoken emerge from masks whose expressions seem fixated in grief or ire (the Cthulhu Mythos features many carven images which represent the Elder Gods and their ardent lust to re-enter the world). The formal Masque, in which faces are covered, often represents a venue which has become stuck in time (masques are commonly found in Dying-Earth tales), or which oppresses reality with its persistence. In Edgar Allan Poe's "The Masque of the Red Death" (1842), masks perform at least two linked functions: the masked Prince Prospero and his courtiers clearly oppress reality with their persistence; but the Red Death itself is a mask, a horrific incursion of transformative truth.
Aspect-maintenance When masks are used as disguises (> Masked Avenger), they tend to serve simultaneously to obscure the face and to present a desired aspect of the disguised persons, enabling them to seem – indeed, in fantasy terms, to become – what they wish. The protagonist of The Mask clearly presents an aspect of his submerged, ideal self, as does Batman – and as does the protagonist of Max Beerbohm's The Happy Hypocrite: A Fairy Tale for Tired Men (1897 chap), who adopts a mask of goodness and finds that it has stuck. In general, the mask as aspect of the whole self serves to streamline that self for dangerous operations, adventures, impostures and roles, passages through portals, and rites.
An anthology whose contents reflect various of these uses is Ray Russell's Masks (anth 1971). [JC]
further reading: Masks, Transformation, and Paradox (1986) by A David Napier, supplying an anthropological perspective; The Mask of Glass (1954) by Holly Roth (1916-1964), nonfiction concentrating on horror.