apple tv widescreen rental not widescreen

what film was it ?
not all SD movies are widescreen. some are 4:3.

If this one was labeled as widescreen in the iTunes store but it doesn't play in widescreen mode, it's possible it's mislabeled in the iTunes store. You might want to contact iTunes store support as they may be willing to give you a credit for another video if it's mislabeled and you're dissatisfied with it.

Welcome to the  Discussion Forums.

By full screen do you mean 16:9 because that is widescreen. widescreen doesn't have to mean 8:3 or any of the cinemascope aspect ratios.

again, can you advise what film it actually was ?

widescreen can be many different aspect ratios, and not just 16:9.
widescreen can take up the full wide area of the tv, it can also have black bars top and bottom, it's all about the aspect ratio of the individual movie.

Still not fully sure I understand you, you say it played full screen, if it filled your screen it was widescreen.

juno definitely does play correctly in widescreen for me.

Check the HDMI settings in your TV and see if there is a setting for screen size and/or signal recognition. If it's set to auto, it may not always be getting it right. Try setting it manually for these movies and see if that solves the problem.