Because DCC tools supports very many options, we resample the animation curve splines, but I think it still need to be interpolated. I think there's some de facto standard for this animation interpolation. Or no interpolation. Can I know what's the mostly used interpolation method?
The most common form of interpolation is linear interpolation.
However, in the case of animating curve splines, another common one is spline interpolation.
In general, you should have an interpolation function f of any kind that satisfies: f(0) = 0; f(1) = 1; f(x) is continuous for all 0 < x < 1 (this can obviously be relaxed if you want jumps); 0 <= f(x) <= 1; and f(x) increasing for 0 < x < 1 (unless you want goofy effects).
Ones that are computationally easier to do are preferred. For example, one may use f(x) = 3x^2 - 2x^3 instead of f(x) = sin(pi x). Both of these have roughly the same shape over the unit interval.
Once you decide on your interpolation function, then simply compute A * f(x) + B * (1 - f(x)) to interpolate between the two values.