Inject Textual Rickrolls
gawk, 316+377 = 693

First command line parameter is the lyrics' filename (375 bytes + 2 for invocation = 377). Rickrolls all other files. Prints to stdout.

BEGIN{FPAT="[^ ]+ *";OFS=""}func d(a){b=tolower(a);sub(/ *$/,"",b);return b}FNR==NR{for(s=$0;NF;$0=s=$0){for(i=1;i&lt;NF;i++){k=k $i;$i="";v[d(k)]="["$0"] "}$0=s;k=$1=""}next}{for(s=$0;NF;$0=s=$0){for(j=NF;(--j)&gt;0&amp;&amp;!(d($0) in v);$(j+1)="");k=v[d($0)];if($0!~/ $/)k=" "k;printf($0 k);for($0=s;j--&gt;=0;$(j+2)="");}print""}


Ungolfed

BEGIN{FPAT="[^ ]+ *";OFS=""}
func d(a){b=tolower(a);sub(/ *$/,"",b);return b}
FNR==NR{
  for(s=$0;NF;$0=s=$0){
    for(i=1;i&lt;NF;i++)
    {
      k=k $i;
      $i="";
      v[d(k)]="["$0"] "
    }
    $0=s;
    k=$1=""
  }
  next
}
{
  for(s=$0;NF;$0=s=$0){
    for(j=NF;(--j)&gt;0&amp;&amp;!(d($0) in v);$(j+1)="");
    k=v[d($0)];
    if($0!~/ $/)k=" "k;
    printf($0 k);
    for($0=s;j--&gt;=0;$(j+2)="");
  }
  print""
}


Test results

Input:

we're
We're
Does he know the answer?
I
how I'm thinking
I'm thinking
I don't know what I'm doing with my life.
Will someone please save me from these memes?
Two strangers walked into a bar.  One said "hello."  The other said "goodbye."
gonna run


Output:

we're [no strangers to love] 
We're [no strangers to love] 
Does he know the [rules and so do I] answer? 
I [just wanna tell you how I'm feeling] 
how I'm [feeling] thinking [of] 
I'm thinking [of] 
I [just wanna tell you how I'm feeling] don't know [the rules and so do I] what I'm [thinking of] doing with my life. 
Will someone please save me from [any other guy] these memes? 
Two strangers [to love] walked into a [lie and hurt you] bar.  One said "hello."  The [rules and so do I] other [guy] said "goodbye." 
gonna run [around and desert you]