Black hole systems can be affected by variable phenomena. This temporal variability scales with the mass of the black hole and it is thus likely that it is related with strong-field gravity. This seminar is dedicated to examining some possible scenarios of black hole variability and link them when possible to future observations. During this talk, I will discuss two kinds of variable phenomena: i) the Galactic center flares of radiation: I will present simulations of astrometric observations of flares by the very-near future (May 2015) infrared interferometer GRAVITY. I will discuss the possibility that GRAVITY could help distinguish different theoretical flare scenarios; ii) microquasars oscillations : I will present a model of microquasar high-frequency quasi-periodic oscillations, namely the oscillating torus model. I will present simulated temporal power spectra for this model and discuss them in the perspective of the 3:2 resonance that some microquasars exhibit.