This question is based on ONCE UPON A TIME. The line "like a fixed portrait smile" suggests that the speaker's smile is—

In Gabriel Okara's "Once Upon a Time," the simile "like a fixed portrait smile" conveys that the speaker's smile is rigid and artificial.

A portrait smile is, by definition, a pose held for a painter or photographer. It is not spontaneous, not responsive to real emotion, and not subject to change. The word "fixed" reinforces this quality: the smile is locked in place, unchanging, and mechanical. It is a social performance rather than a genuine expression of feeling.

The speaker uses this simile as a confession. He has learned to wear this kind of smile in a society where people no longer interact with authentic warmth. Just as others around him have replaced genuine feeling with rehearsed social gestures, he too has adopted a smile that serves a function (politeness, social acceptance) without carrying any real emotion behind it.

The smile is not genuine and emotional (the entire point is its artificiality). It is not artistic and creative (it is mechanical, not inventive). And it is not changing constantly (it is "fixed" - the opposite of changeable). The simile captures the loss of authentic human connection that the poem mourns throughout.