One of the big things that holds a lot of catchers back is their lack of flexibility and mobility. If we aren't able to get into good positions for receiving, blocking, or throwing because of our flexibility or mobility we are already starting at a disadvantage. Improving these two things are imperative for a catchers foundation in athletic performance. While I don't recommend the receiving position you see Contreras in (photo) having the ability to get into a position like that opens up a lot of opportunities to be a good receiver and blocker of the baseball.
So what's the difference between mobility and flexibility? The way I like to look at is this; flexibility allows us to get into further ranges of motion, mobility allows us to perform at those deeper ranges of motion. Mobility encompasses flexibility but also includes the ability to stabilize and perform well at end range of motion.
I have a few catchers that are extremely flexible. They can easily sit low in their primary stances but some of them need to hold themselves into a better receiving position because they are so flexible. They need better stability and by improving their stability in that deep range of motion they'll be able to control their movement and stabilize in better positions. We want the ability to be like Gumby while still having the strength and stability to move and perform in those crazy end ranges of motion.
(If you don't know who Gumby is, you're missing out)
One of the major factors of poor hip mobility is core stability. Often times people like to stretch and stretch thinking that will get them the flexibility they are after. But if you lack core stability your hips will stay locked up in order to protect your body from going into ranges of motion it can't stabilize at. So before you focus on stretching your hips into crazy ranges of motion assess your core stability. If the core is weak, all the stretching in the world won't loosen those hips up.
Mobility is the ultimate goal for catchers, don't just be flexible. Be strong, stable, and Gumby like.