Timeline render preview question

Hi guys.
I admit I just don't understand the timeline render settings.
I have read the articles, watched the videos on how to set up the render settings but I am still having trouble.
Here's the problem. I cannot preview the timeline in the canvas without it chocking.
The only success I have with previewing is when the timeline has a green line above it. I don't know how I got this green line but I can't seem to replicate it.
All my clips are the same Apple pro res 422. My sequence is Apple pro res 422, so is the render control codec in the sequence settings.
Can anyone help me set up FCP so I can play the timeline in the canvas without it chocking?

Thanks for replying.
I changed the RT setting to unlimited and still have only a grey Render line above my timeline not green. I still cannot achieve smooth playback in the canvas.

It depends on the codec you are working with on thew timeline, the number of effects and video layers (compositing), it also depends on the processors of your Mac and the amount of RAM and of course your hard drives.

You did not provide any info of your Mac, the Mac OS, hard drives and ram.

Is your Canvas set to 'fit to window' or smaller?

ProRs (LT) would put less of a strain on your hard drives and processor.

--ken

I have almost no effects in the timeline. Just cross dissolve transitions and a flicker effect.
The Canvas is set to fit all.
I have aMac Boook Pro 2.4 GHz Intel Core Duo. High Speed external Graid dual bus drive with firewire 8 streaming the footage to the timeline.

As I said before the timeline plays ok when it has the green line above but I just don't know how to get it there.
The supposed Grey line that is meant to mean eveything is rendered to play in the canvas does not allow smooth playing.

Not 'FIT ALL'.

Is your Canvas set to 'fit to window' or smaller?

--ken

I change the canvas style all the way through my sditing depending on what I am doing with what clip. No matter what setting it is the same result.

Before rendering you can also click on Option/P to playback your timeline. It won't be in real time but you will be able to see your composites and effects playing.
Steve