In poker there are many contradictions and exceptions to the rule. 
Pro Poker player know that the eyes rarely lie. Many players can’t help but stare at big hole cards, so their length of time peeking is longer. Conversely, if a player is looking to steal the pot, he may look to his left to see if the remaining players, who haven’t yet acted, have quickly glanced at their cards and are likely to fold.
Many pros try to disguise their entire face by wearing a cap, his is to avoid the classic stare-down that poker pros are famous for. They may try to study your face for nervousness.
Physical changes happen including flexing of muscles, eye pupil dilation, palpitating heart rate etc. In poker, when someone has a big hand they are typically ready for confrontation and can exhibit some of these characteristics. .
When you first sit down at a table, study the way the players stack their chips. Although it is a generalization, loose aggressive players typically maintain unorganized.