Adding a category__in to a tax_query

Hey,

I was wondering if anybody nows an easy answer to the following:

I am using a tax_query, but I want to customize it even further by only displaying from a specific category within that taxonomy. Maybe the code will explain clearer what I want:

$args = array(
	'post_type' => 'post',
	'tax_query' => array(
		'relation' => 'AND',
		array(
			'taxonomy' => 'movie_genre',
			'field' => 'slug',
			'terms' => array( 'action' ),
                        // here's what I want to add, only posts of a category within 'action'
                        'category__in' => some_given_cat_id
		),
		array(
			'taxonomy' => 'actor',
			'field' => 'id',
			'terms' => array( 103, 115, 206 ),
			'operator' => 'NOT IN'
		)
	)
);
$query = new WP_Query( $args );

But the category__in simply gets ignored, and if I move it anywhere else my query displays empty.

Where or how can I specify this filtering on the right category.