Add predefined/default values to custom post plugin

You can use default_content filter which filter the default post content on add new post screen only.

Example:

function filter_function_name( $content, $post ) {
    if ( 'movie_reviews' == get_post_type($post) && empty($content) ) {
        $content = "This is some custom content I'm adding to the post editor because I hate re-typing it.";
    }
    return $content;
}
add_filter( 'default_content', 'filter_function_name', 10, 2 );