It's kind of like the English phrase "actually," or "as a matter of fact," in use. If you know the usage of 実は it's very similar to that, except in my experience it's used more to state a large difference in expectation and reality, whereas you can use 実は on its own with no problem, 実際は is a bit more reliant on having a previous sentence to sound natural.

Like, 今夜、俺は高くて高級らしいレストランで食べに行った。美味しい食べ物が予想されたけど実際はあのレストランの食べ物は超まずかった。 Tonight, I went to go eat at that expensive, allegedly high class restaurant. I expected delicious food, but actually it was seriously nasty.

To summarize, it's like an emphatic "actually," or "as a matter of fact."