Mia Sara is undoubtedly a good actress, and she plays Melissa very well, with only one serious problem: she plays Melissa as a good actress. Melissa might be a capable amateur actress, but in the stressful situation in which she finds herself, she is not going to be able to do it, particularly as she is improvising without a script.
We saw the mall scene, in which young Max Walker meets his wife Melissa. We then see the same scene again, but this time she has just been visited by her husband from ten years in the future, who has cryptically pulled her out of the mall and told her a few lies and a few truths and made her very frightened about what is happening--and yet when she then meets the present version of her husband on schedule, she reproduces the scene she has never before played exactly as it would have been had none of this occurred.
The film seems uncertain about its theory of time at this point. On the one hand, it tries to fit future Walker's actions into the gaps, to suggest that he talks to Melissa when she is away from present Walker and might always have done so, and thus that this is the same history as it was; on the other hand, he changes things, such as interfering with the purse snatcher before the purse snatching, which prevents the purse snatching and means that his younger self will not do the same. Everything continues as it did, and for some reason she plays the part exactly the same. A normal person would have been changed by the conversation she had with the older version of her husband; only an actress following a script and pretending to feel what the script says she feels would have lived those hours unaltered by that meeting. Seriously, "Honey, before we make love, I need to tell you two things, first that there are thugs from the future trying to kill us, and second that I'm pregnant." She wouldn't be shaken enough to tell him this? I can't see how they got home from the mall without that being mentioned.
Incidentally, that little incident in which the older Walker stops the purse-snatching skater for what he is going to do winds up being fatal to the film. In the original history, young Walker sees the rollerskating purse snatching and stops the thief. In the new history, the old Walker recognizes the skater as the purse snatcher because he saw the crime when he was younger. Now the younger version of himself will not see the crime, and so when he is older will not remember the purse snatching and will not intervene, and thus his younger duplicate in turn will do so, and we are trapped in an infinity loop, two versions of history each causing the other. It is a little point, but it destroys history completely. (And that is the real reason why time travel is dangerous.)
We have another potential infinity loop staring us in the face, this one threatened by McComb, but we will address that next time.