Theme Entity Reference output I have a content type called SimpleNewsletter which has an Entity Reference field called field_newsletter_content. Multiple articles are linked with one SimpleNewsletter node.   Under Manage Display of SimpleNewsletter, I can manage which fields to display.  

But I have a situation where I only need to display certain fields not all the fields (ex: "Node Title")

In order to theme field_newsletter_content field I have created a new file called field--field_newsletter_content.tpl.php

Is it possible only to display certain fields?
Maybe you could use the teaser for displaying your entityreference nodes. Under Manage display under format for the entity reference field, choose Rendered entity, choose settings and use teaser instead of full mode.

Go the the display settings for the content type that is being referenced and apply your display settings for the teaser display.