wordlist = ['cat','dog','rabbit']
[c for c in word for word in wordlist]

['r',
 'r',
 'r',
 'a',
 'a',
 'a',
 'b',
 'b',
 'b',
 'b',
 'b',
 'b',
 'i',
 'i',
 'i',
 't',
 't',
 't']

[ch for ch in "".join(wordlist)]

[word[i] for word in wordlist for i in range(len(word))]

[c for c in word for word in wordlist]

[c for word in wordlist for c in word]

result = []
for word in wordlist:
    for c in word:
        result.append(c)

In [10]: [c for word in wordlist for c in word]
Out[10]: ['c', 'a', 't', 'd', 'o', 'g', 'r', 'a', 'b', 'b', 'i', 't']

In [3]: [c for c in word for word in wordlist]
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-7-decfecd23a92> in <module>()
----> 1 [c for c in word for word in wordlist]

NameError: name 'word' is not defined
In [4]: word = 'rabbit'

In [5]: [c for c in word for word in wordlist]
Out[5]:
['r',
 'r',
 'r',
 'a',
 'a',
 'a',
 'b',
 'b',
 'b',
 'b',
 'b',
 'b',
 'i',
 'i',
 'i',
 't',
 't',
 't']