Using Silicone Caulking as a Silicone Animatronic mask.
I am not a chemist, but I was thinking of cheaping out on the supplies to go towards the Chucky Doll Animatronic which I am building, as a way to not not spend too much money for Smooth-On or any other brand of silicone, so I got a tube of %100 silicone caulking as the base rubber to start off with, but then I kind of noticed how it will be a little on the stiff side and not like the type of silicone seen as a mask making material. If there is anyone out there who can give me information to adding more "Stretch" into silicone caulking I would be very grateful for some help. Thanks a ton to whoever can help. 
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Silicone caulking, being an air-dry silicone, relies on different chemistry than platinum or tin cure silicones, so I don't think deadening/slacking agents made for those would work with it.
Caulking silicone can be used in a lot of special effects applications, but as you mentioned, it is not as stretchy as other silicones. If you are doing something small like a Chucky Doll you would not need that much silicon, it may be worth it to try a platinum or tin cure silicone with a lower durometer (20 Shore A or below should be fine). Many distributors sell smaller test kits, which may be enough for you. Though with animatronics you often want multiple skins. A test skin to figure out your pigmentation and use while working (which can be hard on the skin) then your final painted skin.
And sometimes you have to make a couple of test skins to get the pigmenting of the silicone to be the right shade/translucency, but with Chucky dolls it should be easy since they are mostly opaque. Just make sure to weigh your pigments or count drops so you can adjust or reproduce the color and density on future skins.
I'd measure the volume of your mold (rice can help with that) and get about 4-5 times that volume (for a smaller mold) so you have enough for 3-4 copies and extra to account for waste.
/Chris