The term reptilian is too all-encompassing to be addressed singularly. There are many alien groups who would fit in this category. We ourselves, the Zetas, have been described as lizards, where we do not have a genetic affinity for what Earthlings know to be lizards at all. Our genetics are similar to humans, from the same base, and we are warm-blooded.
Reptilian alien groups are somewhat cold-blooded, which does not relate to their orientation at all. There are cold-hearted humans, who are warm-blooded, and there are warm-hearted reptilians, who are primarily cold-blooded. Reptilian entities that humans may encounter may be Service-to-Others or Service-to-Self. However, as the reptilian genetics allow for less empathy, or telepathic awareness of others, which the Service-to-Self orientation finds distracting, reptilians most often are Service-to-Self. This is a choice in incarnation. The orientation dictates the genetic nature of the incarnation choice, not the other way around, where the genetic nature dictates the orientation. Thus, on sight, humans should not assume they are dealing with a Service-to-Self orientation when meeting a reptilian alien.
There are numerous reptilians at work in the vicinity of Earth. Some of the forms are humanoid, with arms and hands with fingers and opposable thumbs and legs and feet with toes, eyes forward on the face, rather than facing to the side, and a bi-pedal stance. Others take the form of what you would call snakes, with some development of limbs in varying numbers. Others have forms similar to lizards, and walk on four or more legs, their bodies too stiff to allow a stance. These alien forms can rear up to perform functions, but essentially work at the floor level. Because this places them at a disadvantage when meeting humans, they are not used by the Service-to-Self orientation in addressing humans, as the Service-to-Self orientation uses intimidation to function.
A reptilian form familiar to humans is that termed by humans, Dino. Where not as large as the Earth's dinosaurs, this reptilian looks like a miniature Tyrannosaurus Rex. This form is used by the Service-to- Self orientation in encounters with humans, as intimidation is supported not only by the imposing form, which stands at least as tall as the average human, but by educated humankind's awareness of the fierceness of a similar dinosaur in Earth's past.