For sleeping and sexual activities one often lies down. For most activities which does not involve moving, sitting is usually preferred, e.g. reading, watching television, using a computer; this also applies for moving in a vehicle.
Standing and squatting is mainly done when there are not enough seats, e.g. in a public transport vehicle, a train station, a bus stop, a waiting room; whether people will sit anyway depends on the availabilty of other places to sit (including enough space on the floor), how inventive one is, how conventional, how dirty these places are, how dirty one is willing to become, and whether paper etc. is available to sit on (these things also apply when there are seats, but dirty).
Standing in a moving vehicle is less stable than sitting and usually requires holding on to something to absorb accelerations (going faster and slower and making turns); for this poles and/or handles are often fitted. Squatting may be difficult because of being too unstable.