Expression doesn't have a strict 'correct' answer for AI transformation quality, but it does affect the mood and believability of the final result in ways worth considering.
Expression carries into the result
Because face preservation aims to keep your actual expression and features, a natural smile or neutral expression in the source photo tends to carry through into the transformed result — an exaggerated or unusual expression may look somewhat unnatural once combined with a dramatic action pose.
Matching expression to the desired mood
For a celebratory, energetic sports-photography style result, a more animated source expression (a genuine smile, an energetic look) tends to blend more naturally than a completely flat, neutral expression.
The practical takeaway
A relaxed, natural expression — genuinely you, not a forced or exaggerated pose — generally produces the most convincing and flattering results, since the model has an authentic baseline to work from.
Does closing your eyes or looking away from the camera hurt results?
Generally yes — a clear, open-eyed, camera-facing (or near camera-facing) source photo gives the model the clearest facial data, so avoiding closed eyes or looking away is a reasonable practical guideline.