I’ve watched and rewatched that lesson, Tuomo has done a fantastic job ! Also i’m very interested in that lesson, and that also might be usefull to copy some of his vocings for arrangement, and playing. At 13:05 in the video, he plays a Bbo7 and a Bbmaj7, instead of only playing a Bbmaj7, why ?
-Guillaume-
Thanks @Guillaume,
I’m glad you liked the lesson!
About the Bbdim,
A tonic diminished chord that resolves to a tonic major is a common harmonic movement.
As we know, diminished chords usually function as dominant chords, or at least as something that needs to be resolved.
Let’s stay in the key of Bb. Bbdiminished chord can function as an A7:
A7, of course is a dominant to D (in the key of Bb D-), which can be treated as a Bbmaj7 as well:
This is why Bbdim → Bbmaj resolves nicely, even if the diminished chord is not the usual dominant diminished chord.
Also this is why a usual reharmonization for the tonic dimished is 25, in this case E-7 A7 → Bbmaj7.
Here couple of examples of tunes that this occurs:
I Remember you in Fmaj:
Whispering in Eb:
And many others.
Let me know if this helped, and let me know if you have any further questions!
-Tuomo
Another wonderful arrangement by the Master, Kent Hewitt did a video on this, that might be a good supplement to the lesson of @Tuomo on Bill Evans, for those like me whom are absolute fans.
–Guillaume–
Hey all, im super interested in getting more and more into Bill Evans style, but, i don’t know where to start, there’s just so much to do, do you have any advice ?
Thanks in advance
–Guillaume–
Hi Guillaume 
I can recommend some theory topics which can be commonly heard in the playing of Bill Evans.
Moving from easier topics to harder topics:
Listen to all of the inner voices Bill uses in the head of “Like Someone In Love”: 
Study the following lesson, and then perhaps try to work out some of those inner voices he is playing in the recording above.
After watching Tuomo’s lesson on Bill Evans’ style, you will have seen how Bill Evans uses Sus chords and suspended harmony.
We have a lesson on that topic here:
Bill Evans was a master of drop 2 voicings.
There are some nice examples from 1:55 onwards in this recording:
Here’s is our course on block chords and drop 2 voicings:
Just use the above as guidance Guillaume.
There will be something about Bill Evans’ style which speaks to you.
My main recommendation is to figure out exactly what that thing is.
Find examples on his discography with specific things that you want to be able to play, and then spend the time to transcribe and emulate his playing.
Any further questions let me know. 
Aside from the harmony topics @Hayden listed, I can recommend working on rhythm as well, and in particular to become comfortable with hemiola in your melodic improvisation. Bill Evans often said that he worked hard to liberate his melodic improvisation from “the bar line,” and I know from my own attempts that it’s challenging to do.
Start with simple melodic phrases that you choose from your own improvisations or that you transcribe from Evans’ examples, and practice playing them starting on every eighth note (every half beat) as you comp for yourself and keep steady time with a simple, fitting chord progression in the left hand.
Having his mind work with great facility “across the bar lines” is a hallmark of Bill Evans’s playing, and the better you are at this, the more of his sound you’ll have access to in your own improv.
