In general, use the slowest film that the lighting situation will allow, especially when using 35 mm. Fast film is used in reportage situations where the light is very dim, in action shots and handheld telephoto shots where fast shutter speeds are necessary Film Characteristics Colour Sensitivity Film emulsions are sensitised during manufacture to some or all the colours of the spectrum. A coarse grain pattern will be rough in texture and will break up the delicate tones when enlarged. Slow film has a slightly harsher contrast than faster films - therefore development times are less for slow films as this reduces contrast. If transparency is out by as little as a third of a stop either way, reproduction will not be true to the original subject matter.
Some photographers get better results by manipulating exposure and development time.
It is a good idea to bracket exposures when working. This is measure by the eye and means the ability of the film to record fine detail. There is a chart used to calculate this - film data sheets give this information. As grain decreases, the other two increase. If dense, then film is overexposed, if thin, then film is underexposed.