What are two similes Montag uses to describe Clarisse, and what purpose might they have other than characterization?
Remember that a simile is a comparison between two things, often incorporating the words "like" or "as"; for example, "quiet as a mouse" or "sly like a fox".
After meeting Clarisse, Montag reflects on her and what their conversation has meant to him:
She had a very thin face like the dial of a small clock seen faintly in a dark room in the middle of a night when you waken to see the time and see the clock telling you the hour and the minute and the second, with a white silence and a glowing, all certainty and knowing what it has to tell of the night passing swiftly on toward further darknesses but moving also toward a new sun.
We can draw some characterizations about Clarisse; that she is calm, quiet, informative, but also that there is a sense of great danger coming ("further darkness") as well as happiness (the new sun). This might imply that Clarisse is "waking Montag up" - informing him that he is surrounded by evil, but that good things are to come.
A second line is even more straightforward in its use of simile;
How like a mirror, too, her face. Impossible; for how many people did you know that refracted your own light to you? People were more often-he searched for a simile, found one in his work-torches, blazing away until they whiffed out. How rarely did other people's faces take of you and throw back to you your own expression, your own innermost trembling thought?
There is more of a social commentary here than a commentary on Clarisse, although Montag is at least considering how she has caused him to reflect upon himself. The deeper meaning is that so many people are concerned about themselves, their own thoughts, efforts and purpose, that they don't take the time or care to acknowledge or articulate someone else's existence. Instead, they go on like robots or batteries until their energy runs out.
Montag goes on with other similes, considering that Clarisse is, in a sense, better at being human than he is; she has a deeper appreciation for language and insight and seems to be very much in control of herself and, in a way, of him too. We might also detect a bit of jealousy and defensiveness; he suggests Clarisse is watching a marionette show, with people like himself as the marionettes. This, and the comparisons to inanimate, mechanical objects, might imply that Montag feels threated by Clarisse and is imagining her as an unfeeling "outsider" who isn't really human.