a couple

The Publication Manual of the American Psychological Association (1994):

If the action of the verb is on the group as a whole, treat the noun as a singular noun. If the action of the verb is on members of the group as individuals, treat the noun as a plural noun. The context (i.e. your emphasis) determines whether the action is on the group or on individuals.

The bridal couple is surrounded by well-wishers.
[Here, the couple is regarded as one unit] BUT

The couple have separated.
[Here, the couple are regarded as two separate people.]

In your example, however, neither one seems more natural to me. I believe both are technically acceptable, so use the one that sounds better in the specific context.

There is a couple leaning on the wall of the building. Cut out the fat.

Makes sense, Tim.

And I would if I could, peon, but it was written by a Korean student and they won't take "rewrite" as an answer.

The decision has already been decided in the first two words "There is". It would be improper to switch from the singular 'is' to the plural 'are' mid-sentence. That said, the resulting sentence is incredibly clumsy and will still sound "wrong" to most native speakers I believe.

There is a couple leaning on the wall.

It's not a very good example but isn't it:

There is a couple, who are leaning against the wall.

There is a couple, while they lean against the wall, gaze longingly into each other's eyes.

Is this an example of a suborindate clause being plural and the main clause being singular?

There is a couple who are leaning against the wall.
or
There is a couple that is leaning against the wall.

I think that by using the word "who" you are choosing to refer to the two seperate people in the previously mentioned couple.

There are many similar (collective) nouns, e.g. police, audience, congregation.
If you're thinking of the couple as a UNIT, then IS. If you're thinking of them as two INDIVIDUALS, then ARE (native speakers are very confused about this anyway!! Just to throw some sand into people's eyes, what about "Everybody's here, AREN'T THEY?" - and somebody, nobody, anybody... plus the trouble we have with "somebody's left THEIR / HIS / HER books behind...)
Happy New Year!

Well that's what's so irritating about this.

The sentence has two parts:

1) There is a couple (collective identity)
2) The couple are leaning on the wall (two individuals)

I seriously don't think that "The couple is leaning on the wall" can ever be appropriate.

Why not just say 'The couple is leaning against the wall'?

I think Tristan has a point about using "who" to refer to the couple. I humbly recommend, "There is a couple which is leaning on the wall of the building".

I'm sorry, but in my view you can hardly ever use "which" when dealing with people, and definitely not in this case. And, in all but very few cases where one can think of them as a single unit, it should be in the plural.

In informal speech, it would be perfectly acceptable to say "There's a couple (over there) who are leaning on the wall."

But I think that's partly because "There is/There's" has become a standard introductory phrase that speakers frequently forget to pluralise (because they haven't yet necessarily realised what's coming afterwards).

Similarly with "Where is/Where's". It's quite common to find oneself saying something like "Where's my shoes?", although strictly speaking it should be "Where are..."

However, it would also be natural to say: "There are a couple (over there) who are..."

In my view, anything like "A couple is leaning against the wall." sounds just plain wrong.

Even, "The bridal couple is walking back down the aisle." or "The couple has a wedding list at Selfridge's." sound wrong too, even though one could argue they are being treated as one unit. Instinctively, even in such cases, I am thinking of two people, so I expect a plural.

However, other examples such as "My family is very old" (We can trace our ancestry back to William the Conqueror) work. This contrasts with "My family are very old" (We're all elderly and there's no younger generation).

A couple of people are...
the couple is...

http://www.flocabulary.com

The couple is. Period. "...of something" is implied.

Ah yes, the old debate over whether collective nouns are singular or plural.

So fat this hasn't been mentioned: In the US collective nouns (couple, team, group, band, etc.) tend to be considered singular, while in Britain for they most part they are considered plural.

One big problem with "There is a couple, while they lean against the wall, gaze longingly into each other's eyes." is that "couple" is not the subject of the sentence. It's even debateable whether there is one. In order to be correct, this sentence would have to be:

The couple, while they lean against the wall, gaze longingly into each other's eyes.

or

There is a couple, leaning against the wall, gazing longingly into each other's eyes.

I also agree that a collective acts as a singular when it refers to one unit:

The couple is sitting in the park talking.

and that it is plural when they are individual:

There are a couple of people in the park.