But don't take the idea of 'location' too literally. Yes, you will be facing each other over a table; with the beer sitting on the table in front of you both. But what is implied is that the discussion/conversation will last at least for as long as it takes to drink at least one glass of beer. It's a casual invitation to discuss some matter in detail, but informally. You could be there having a pleasant time till the pub closes!
It also does not literally require a brewed malt beverage. Suggesting or asking that something be discussed "over a beer" indicates a desire to discuss it in a friendly, non-confrontational manner, like friends drinking beer together.
The expression is not specifically "over a beer" but, rather, anything to eat or drink that implies the friendly nature of the setting. Thus, one might say, Let's talk about this . . . over a glass of wine or over lunch or over a cup of tea.
Can we use this idiom for other kind of activities such as watching a movie?
In my native language (italian) we use <<."davanti ad una birra".>> that literally means "in front of a (glass) of beer" and this is useful for many situations as discuss, watching tv and so on.