1.) Pet your horse: Hold 'Jump' while approaching Roach

2.) Roach goes a little deeper in water

3.) Roach has couple little extra animations here and there (looking at you, raising foot while being petted, maybe raising tail/neck in water?)

4.) Roach walks backwards now

5.) While mounted you have a new key (instructions on how to use it underneath here)

Navigate to your 'My Documents \ The Witcher 3' folder
Open up the file 'input.settings' with a text editor like notepad++
Copy paste the following at the top of the document

[Horse]
IK_Alt=(Action=HorseKick)

Make sure all capital letters are capitalized above and save and exit file

(By the way you can change IK_Alt to any key of your choosing, that is the Dodge button by default)

5a.) To Rear your horsey: While Riding Roach Press and hold the HorseKick (Dodge by default) key or whatever key you chose above while your camera is facing forwards on Roach when you are both standing relatively still.

5b.) To Have Roach do a Kick while mounted press and hold the Dodge (or whatever key you bound in the intructions above to HorseKick to) while having the camera face anywhere in the 180 degree cone facing backwards while on Roach and standing relatively still.

6.) Roach Kicks enemies in battle with or without you mounted I believe.

7.) Roach does damage to beings you run into while riding him now

8.) Roach looks at you sometimes if you whistle and/or just randomly on his own I believe.

9.) Your verbal commands while riding roach are reduced a bit by default and possibly has a new 'chill' command I believe.

10.) Roach has an extra speed variable of SLOW WALK (and possibly improved blending movement animations) that may make things feel more natural

11.) So far merging the file hasn't proved problematic on my personal load order but this is a simple fast release so we'll see how that goes.

12.) Originally he had a new slowwalk-toggle-all-walks button but I didn't incorporate it.