Crafting a believable alien in written form starts with the point of view. If the POV is coming from a human or pre-established non-human character, their view of the alien is what the reader needs first. Even if it differs from the writers knowledge of the alien character.
For example, if the alien comes across as gentle and non-threatening to the main character those are the characteristics that are highlighted in the description. A full description leaves less to the imagination and complete descriptions lend to the assumption that there is nothing to hide.
If the alien is seen as threatening then a limited number of threatening features are highlighted. Leaving more up to the readers imagination increases the intensity of the scenes (i.e. not seeing the shark in Jaws or the full Alien in the movie right away). Describing a haunting gaze with a gravely voice allows the readers to pull their own fears and nightmares into the story and connecting with it on a level that would be difficult to achieve otherwise.
I wrote a poem a while back that walked the line between the two.
He lurks around corners
in the background
where no one will notice
eyes piercing the darkness
His presence is felt
seen only in glimpses
He is a shadow himself
he guards those who walk in gray
who are as much light as dark
who balance between seen and unseen
He keeps us from falling into blackness
and gently pulls us from the blinding light
His presence first met with fear
is now reluctantly welcomed
A shadow guardian
lives a lonely existence
on the fringe of society
because He is there
© Graylin Fox 2010