What is the difference between casual observers and scientific observers?

There are different types of observers. Casual observers differ from scientific observer. Seeing and observing are two different things. Not that everybody seeing is actually observing the thing.
A casual observer just sits back and watch for something to happen while a scientific observer comes up with some hypothesis to work on it. 

Observation is a central theme of scientific investigation. Scientists are usually expert observers, who have sharpened their senses when it comes to observing the external world and its phenomenon. The principal difference between a scientist and a casual observer is that the scientist ponder on his observation and analyse it critically to get more insight and information and the observation may lead to a totally new experimental project. While a causal observer will just observe and move on.