Brettanomyces comes in many forms, leading to many different flavour profiles. The main three you will come across commercially are:
- B.Claussenii - Fruity with mild funk
- B.Bruxellensis - Tasty Horse Blanket (this is Orval)
- B.Lambicus - heavy funk with sour fruits
Depending on when you add the Brett to the fermentation will determine how soured your beer gets from the Brett. Brett operates in 2 modes, aerobic and anerobic and takes about 24-48 hours to transition between the 2. In the aerobic phase you will get higher levels of acetic production than in the anerobic phase [https://link.springer.com/article/10.1007/s00253-002-1197-z ]. You would be adding the B.Brux to the FV after primary fermentation is completed and therefore it would enter in an anerobic state and produce very little acetic acid.
If you wished it to make a sour you could put a porous bung(cork) in your carboy to allow oxygen exchange over the 40 days, which would allow the brett to metabolise the remaining complex sugars with some O2 present allowing for more acid production.
But, realistically for a real sour Sour, you either would want to do a kettle sour, or be adding the Brett at the start of fermentation ie with oxygen present, or adding it along with a mix of bacteria Lacto, Pedio, etc...
A great source of information about sours, brett, mixed fermentations is the Milk The Funk wiki:
And, above all don't fear the funk, just be ready to clean everything properly, and enjoy the funky flavours.