Ben Affleck tends to wear his emotions on his sleeves -- and he seemed to be doing exactly that at Jennifer Lopez's premiere ... where he looked a bit fired up while talking to his wifey.

The famous couple showed up in their designer duds to the Dolby Theater Tuesday night for J Lo's "This Is Me ... Now" flick/music video -- and everything seemed pretty hunky-dory at first. They posed for pics, smiled and went about their business like normal ... all smiles.

However, at some point while inside ... the mood appeared to change, and so did Ben's.

Check out this video, obtained by TMZ ... J Lo and Ben were having a private convo in the lobby of the large, crowded auditorium -- and it looks like the two of them were engaged in a pretty engrossing chat, at least from Ben's side anyway ... dude had a lot to say.

Hard to say what exactly he might've been communicating here -- you could say he looks a little annoyed, but he also just comes across as spirited. Jen, meanwhile, is totally chillin'.

Eventually, she turns away and talks with others ... while Ben continues to speak with a guy.

As for the movie experience itself ... everyone seemed to love it and was really into seeing J Lo's life story on the silver screen. The leading lady herself even got onstage and gave a speech, bringing herself to tears as she talked about producing the biopic, of sorts.