The problem with some films is that dialogue is mixed at a playback level that is extremely loud. At lower volumes, the dialogue therefore sounds unclear.
Acoustically ceiling to floor reflections are meant to blur dialogue, so tilting your speakers downwards can help. Turning on dynamic range compression may be useful if you don't listen at reference volume levels.
If you're interested in getting very clear dialogue and all round good film soundtrack performance, the THX, Dolby Digital, and DTS speakers of choice are made by M&K (Miller and Kreisel). THX certified centre speakers by M&K (and generally) are designed so that the dispersivity is such that less sound is radiated up and down. This reduces ceiling/floor reflections and improves dialogue clarity.