The effectiveness of this movie poster is its use of color contrast (saturated reds with very unsaturated, dull, grays, whites, blacks) to draw attention to the very center of the piece. This poster also uses lines to its advantage, many vertical lines on the wall and floorboards keep the attention moving back to the center, as well as the diagonals from the window drawing attention back to the room. The lines have a circular motion to them always bringing attention back to where it needs to be. Even Sweeney Todd's pants have vertical and diagonal lines to keep the focus moving.
The type then is red for the tagline, making it pop, and it is clearly visible because it is right down one of the vertical lines so our eyes wont miss it. Then the "Johnny Depp is Sweeney Todd" has a very large font size, with more of a silver to make it pop more than the other colors, and the use of the red horizontal and red splash on the name draws focus to it more because the line is a contrasting direction to everything else. It has a visual hierarchy of large object (chair, Todd), lines bringing focus down to tagline, then down more to the title and other information.
I really liked this poster because when I first looked at it I thought it was very simple, but also incredibly eye catching, but the more I learned about why it was eye catching, the more I realized it was quite complex.