"we already have watched his film" gives the same semantic meaning. What is wrong with it? I agree that "We have already..." is "better" English ;-)
The difference is really subtle. “Посмотрели” implies that someone had recommended the film to you or made you promise that you will watch it. «смотрели» is used when you just want to inform somebody that you have seen the movie rather than confirm the fact. Another meaning of «мы уже смотрели» is “we were already in the process of watching”. This second meaning can only be expressed when the sentence is followed by a time clause. The implication will be «мы уже начали смотреть».
Thanks for the reply. In that case, посмотели seems to correspond to the British English "We watched", because we are thinking of a specific time, as in your example (it could also be when the film came out, or because it was on TV last night). "We have watched" could be at any time in the past. It would be interesting to know what you think.
In fact, «мы посмотрели» can be either “we watched” or “we have watched”, but, in the case of «мы уже посмотрели», only the latter is an option. Whether the Russian verb is perfective (посмотрели) or imperfective (смотрели), the presence of «уже» indicates the need to use Present Perfect in English. «Мы уже посмотрели» implies «You’ve been wondering if we have watched it - yes, we have”, whereas «Мы уже смотрели» will be appropriate as an answer to someone’s invitation to join them for watching the film. The implication may be that you have already seen it, but it’s been a while since you did.
In American English, I know of no one who says "we've already watched this film". Everybody says "we've already seen this film" - - similarly, most people say "let's go see a movie" and not "let's go watch a movie" so... Can this be added as an acceptable answer? The other is awkward.