Engagement is a very important aspect to consider indeed, and I would encourage to try to engage players by their work. Taking a genuine interest in people's characters and other creations. In the end of the day, that's what we're here for, and that's what we pour our blood sweat and tears into. Don't get me wrong, as others have said engaging with other players OOC is important, but don't overlook this step. Play to the player's ideas and character: If you're a GM try seeing how you can integrate their personality into your hooks, and their backstory into your plot. Try to see how you can [b]personally[/b] engage their character in a unique way or role that they aren't just filling as a token spot that anyone could take.

If you are a player, then I recommend finding a dynamic for the characters to establish. This can but doesn't have to be prebuilt,but make use of the other player's character sheet to find a way for your character to organically get their attention if they are alone, if they are not alone or not passive, then instead try engaging with what they are trying to do. Pre roleplay you can also try to define predefined relationships between the characters.