Auto Fill Content Fields I want to have a content type named person with fields like first name, last name, phone number ....

In another content type named account, it will also contain the fields belong to person, but I want to have a drop down list for the field person in account to select from existing person content, and auto populate the rest of the fields in account.

Also, is it possible to auto populate a field based on the value in previous field? For example, two fields of the same content type, user id and user email, while user input the user id sunnyside, user email would automatically generate sunnyside@gmail.com.

Thanks
you have to write alter function for your form then you may attach fields from another entity by using field attach form function here is a example for loading profile2 fields form elements 

 global $user;
         $profile = profile2_load_by_user($user,'profile');
        field_attach_form('profile2',$profile,$form,$form_state,'und');