The biggest difference between playing from light rough and thick rough is the affect that distance will have on the ball. Typically when playing from light rough, we have to assess if we are going to confront the possibility of a 'flyer' lie. This is when grass gets between the clubface and the ball causing the ball to have less spin thus sending the ball further than normal.
If the grass is obviously going to come between the clubface and the ball, start with choosing a shorter club. Typically you would likely go one to two clubs shorter depending on other factors such as wind. Since you won't be creating too much backspin from this lie, you also want to have the correct strategy on which portion of the green you would like to hit toward. It may even be safer to keep the ball short of the green as if there's trouble over the back, it would increase the likelihood that you may find this penalty area.
If the grass is short to an extent that you feel grass won't come between the clubface and the ball, then you can play your standard shot with the appropriate club. In terms of the swing when playing from light rough, have the ball in the standard position which would be just forward of middle for the irons and swing a little steeper than normal. This will enable you to catch the ball first rather than the grass decreasing the likelihood of causing a 'flyer' lie.
If you'd like any further help in navigating through different conditions in the rough, contact me for details.