Referencing array index inside optionally unwrapped dictionary

It turns out that its a little more complicated than I thought...

I thought it might be something like:

if let dict = movieDictionary["Spectre"] {
  if let leadActor = dict["cast[0]"] {} 
}

However I'm stuck on this Bummer:

Bummer! Make sure that you're using array subscript syntax to get the value for the lead actor

But after doing a some research I came across several examples that uses a question mark, so I tried:

if let dict = movieDictionary["Spectre"] {
  if let leadActor = dict["cast"]?[0] {} 
}

More research brought me to this stackoverflow thread:

http://stackoverflow.com/questions/27609430/swift-optional-array-index-error

If you read through the first answer in that thread (about "3 possible things here that you might be conflating")

and you'll see that things are a bit more complicated in terms of options than Pasan hinted at in any of the videos.

I found very few examples anywhere on the Internet where a value for an array inside a dictionary was referenced using an array index (using the optional-oriented "if let").

Note: If I drop the 'if' from the second 'if let' I get a different error:

Bummer! Make sure that you're using optional binding to unwrap the value obtained from using the cast key

..so I'm thinking 'if let' is necessary on both lines.

Also tried:

if let dict = movieDictionary["Spectre"],
  if let leadActor = dict!["cast[0]"] {
}

..which gave me this Bummer:

Bummer! Make sure that you're using optional binding to unwrap the value obtained from using the Spectre key

..which just seems really wrong.

So I guess I'm stuck for now...