Assuming you are mixing the entire album…..
If the band has live drums, then it is somewhat important to try and make the drums somewhat consistent from song to song. They don’t need to be exactly the same, but often that’s not desireable. But because live drums are such a complex instrument and because there is such a MASSIVELY WIDE VARIENCE in how they can sound depending on how you mix them – far more than any other instrument, you do need at least some cohesiveness. In a situation like that, I will copy drum channels from one session to another to use as a starting point on the other songs and then tweak from there specific to the song.
If there aren’t live drums, then all bets are off of course.
Each mix engineer has little things they do that make up their sound regardless of genre. That in and of itself will lend a certain amount of cohesiveness to the album. Beyond that, the direction a mix engineer gets from the artist/band/producer/A&R will go a long way. The better the vision, the easier it is to stay on track. A lot of the time all that direction is emmidiately accessible simply by listening to the tracks. Other times you need notes and conversation.
One thing I definitely do is try to come up with a certain signature for the artist. It might be certain kinds of vocal delays that I use across most of the songs. Or they way I pan stuff. Or anything really. It’s generally not the kind of thing where I’m using the exact same effects. It’s more of an attitude.
In mastering, if everything is mixed by the same mixer, then it’s relatively straight forward provided there are no screw ups. But when you have records mixed by different people, which is very common these days, then obviously the mastering engineer will need to do some EQ here and there to make sure there are similar amounts of bass, midrange, etc. in each song. The reality is that there is wide variance from record to record as to how much bass there is or the overall EQ shape (if you want to think of it that way). And each mix engineer may be using a different reference point for how much low end should be in there or how bright the record should be. Part of the mastering engineer’s job is to balance that all out so that it’s a consistent sound from song to song.
This all said, because we now live in a singles world where album sales don’t mean much, it begs the question of whether or not it’s important to have a cohesive sounding album. Certainly a lot of mainstream albums of recent have zero cohesion because they feel it’s simply not important since most people won’t be listening to it as an album. Even if they buy the album instead of just some singles, chances are they won’t LISTEN to it as an album, but will instead mix the songs up with other songs in a playlist or on random or something. Personally, I like to listen to entire albums, but I’m also well aware that I don’t remotely represent the average consumer.